Several techniques have been suggested for producing nanostructures, within the field of making e.g. hard drives and electronic circuits. One particular technique which has drawn a lot of attention in recent years is so-called nanoimprint lithography for producing nanostructures, i.e. structures in the order of 100 nm and smaller. In the main steps of such lithography, a pattern of nanostructures is transferred from a stamp to an object. The object comprises a substrate and, applied thereto, a film of e.g. a polymer material, often called resist. After heating of the film to a suitable temperature, the stamp is pressed into the film. The stamp is then released from the object when recesses of a desired depth have been formed in layer. Thereafter, any remaining film in the recesses is removed, for instance by etching, thereby exposing the substrate. In subsequent process steps, the pattern in the film is reproduced in the substrate or in some other material which is applied to the substrate.
A related lithographic technique is photolithography. A photolithographical process typically involves the steps of coating a substrate with a photoresist material to form a resist layer on a surface of the substrate. The resist layer is then exposed to radiation at selective portions, preferably by using a mask. Subsequent developing steps remove portions of the resist, thereby forming a pattern in the resist corresponding to the mask. The removal of resist portions exposes the substrate surface, which may be processed by e.g. etching, doping, or metallization. For fine scale replication, photolithography is limited by diffraction, which is dependent on the wavelength of the radiation used. For fabrication of structures on a scale of less than 50 nm, such a short wavelength is needed that the material requirements on the optical systems will be major.
A different form of lithography is radiation-assisted imprint, which has been presented as step and flash imprint lithography by Willson et al. in U.S. Pat. No. 6,334,960. Similar to the imprint technique briefly described above, this technique involves a template or stamp having a structured surface defining a pattern to be transferred to a substrate. The substrate is covered by a layer of polymerisable fluid, into which layer the template is pressed such that the fluid fills recesses in the pattern structure. The template is made from a material which is transparent to a radiation wavelength range which is usable for polymerising the polymerisable fluid, typically UV light. By applying radiation to the fluid through the template, and potential post-baking, the fluid is cured and solidified. The template is subsequently removed, after which the pattern thereof is replicated in the solid polymer material layer made from the polymerised fluid. Further processing transfers the structure in the solid polymer material layer to the substrate.
A problem related to radiation-assisted imprint is that either the template or the substrate has to be transparent to radiation of a wavelength usable for curing the polymer layer. This involves a limitation in selectable materials. When the purpose of the process is to provide a structure for e.g. electric circuits, the substrate is often made of silicon, or some other form of semiconductor material, which typically are opaque to UV radiation. This means that the template needs to be formed of a transparent material, such as quartz or SiO2. However, such templates are expensive to produce, and they are also very sensitive to wear.